Gas chromatography (GC) relies upon the chemical equilibria of analytes between a mobile phase and a stationary phase in a GC column to bring about a temporal separation of the analytes in a gas mixture into a series of elution bands. An ideal gas chromatographic column has a high resolving power, high speed of operation, and high capacity. Most current methods of gas chromatography rely on an open capillary tube with a stationary phase coating the inner wall of the tube to generate chemical separations. These columns are typically made of high purity synthetic amorphous silicon dioxide (fused silica) or borosilicate glass capillary tubing. See R. L. Grob and E. F. Berry, Modern Practice of Gas Chromatography, 4th Ed., Wiley-Interscience (2004). However, recently there has been interest in metal capillary columns, because metal columns withstand elevated temperatures, endure buildup of substances with high boiling points, and have a large sample capacity. See Watanbe et al., “Features of Metal Capillary Columns”, J. Microcolumn Separations 12(6), 345 (2000). As an alternative to the open capillary column, the column can be packed with a support that is coated with a stationary phase to achieve chemical separations. Regardless of the column type, to achieve good column performance requires a stationary phase that is selective of the analytes to be separated, has a high coating efficiency, is chemically stable, and has a wide operating temperature range.
Separations are the result of selective analyte-stationary-phase interactions and differences in the solubility of the analytes to be separated. Analyte-stationary-phase interactions include ionic, charge transfer, dipole-dipole, and hydrogen bonding. A stationary phase with like functional groups will provide a stronger interaction with an analyte having a similar functional group, enabling greater retention and resolving power. Therefore, the stationary phase preferably comprises functional groups that are like those present in the analytes to be separated. Consequently, nonpolar stationary phases primarily separate analytes on the basis of boiling points (vapor pressure). Conversely, a polar stationary phase can be used to separate polar analytes.
Coating efficiency (i.e., how well the column or support is coated with the stationary phase) depends on the column type, coating method, and stationary phase material. In general, retention and sample capacity tend to increase with increasing stationary phase thickness, but at the cost of column efficiency (i.e., theoretical plate number). For wall-coated open columns, the stationary phase coating preferably comprises a relatively thin film of the stationary phase material uniformly coated on the inner wall of the column. A thin, uniform coating enables a short and consistent residence time of the mobile phase analyte in the stationary phase, enabling sharp band definition and a narrow retention time distribution of the eluting analyte molecules. Therefore, thinner films can provide higher resolution for high-boiling point analytes. Thicker film columns are inherently more inert and can be used with more volatile analytes or for separations that are performed at lower column temperatures.
In addition, the stationary phase should be chemically stable and not react irreversibly with the mobile phase. Especially for temperature programming, the stationary phase material should have a wide operating temperature range and have a low vapor pressure at elevated column temperatures. In particular, the presence of residual volatiles and low-boiling point species in the stationary phase material can cause the column to bleed and contaminate the detector. Conditioning the column by purging the column at elevated temperature with a high-purity carrier gas prior to using the column for analyses can remove the residual volatiles and extend the column lifetime.
With fused-silica columns, the fused silica surface is pretreated prior to wall coating to deactivate the silanol surface sites and to create a surface that is more wettable by the stationary phase. The deactivated column is then coated with a uniform thin film (e.g., less than 8 μm thickness) of the stationary phase material on the inner wall of the clean, de-activated fused-silica tubing. Typical stationary phases used with fused-silica columns include polysilioxanes and polyethylene glycol phases. Polysiloxane phases can have high solute diffusivities and excellent chemical and thermal stability. Furthermore, polysiloxanes can have a variety of functional groups that exhibit a range of polarities. Polyethylene glycol phases typically have high selectivity for polar analytes. Depending on the material, stationary phases are typically prepared by solvent evaporation or solution-coating methods. The stationary phase can be further immobilized to provide greater chemical and thermal stability by in-situ crosslinking and/or chemical bonding to the fused-silica column. For example, the fused-silica column can be coated with an OH-terminated polysiloxane and heated to an elevated temperature to initiate condensation reactions between the hydroxyl terminations and the surface silanols of the fused silica to achieve a tightly bonded phase.
Recently, portable, handheld microanalytical systems, which have been termed “chemical laboratories on a chip,” have been developed based on gas chromatography to enable the rapid and sensitive detection of particular chemicals, including pollutants, toxic industrial chemicals, high explosives, and chemical and biological warfare agents. Both open and packed in-chip channels have been used with current GC-based microanalytical systems. In particular, etched silicon channels are commonly used for microfabricated GC columns. Anisotropic wet etching or reactive ion etching can be used to form high-aspect-ratio rectangular channels with precisely controlled channel depth and width in a substrate. Typically, rectangular channels are about 10 to 100 microns wide and about 200 to 400 microns deep etched in the surface of a silicon wafer. For dense packing, the channels typically have a spiral or serpentine pattern in a die that is approximately one square centimeter in area. Overall column length is typically about 1 meter for open channels and as short as 10 centimeters for packed channels. Such high-aspect-ratio rectangular channels can provide relatively high column efficiency combined with relatively high volumetric flow rates and high stationary phase surface area. See C. M. Matzke et al., “Microfabricated Silicon Gas Chromatographic MicroChannels: Fabrication and Performance,” Proceedings of SPIE, Micromachining and Microfabrication Process Technology IV, 3511, 262 (1998); and G. Lambertus et al., “Design, Fabrication, and Evaluation of Microfabricated Columns for Gas Chromatography,” Anal. Chem. 76, 2629 (2004); which are incorporated herein by reference.
However, silicon-based microfabricated columns are expensive to process, require highly specialized flow interconnects, and have flow and separation limitations due to channel cross-section and length. Therefore, microfabricated columns manufactured from materials other than silica and glass, especially metals, are being developed. These alternate column materials can enable columns that are easier and less expensive to fabricate, can provide enhanced durability and strength, and yet can provide the chemical inertness of silicon.
Non-silicon columns can be fabricated out of virtually any material using a variety of micromachining techniques. For example, the GC column can be fabricated using a LIGA process (LIGA is the German acronym for Lithographie, Galvanoformung, and Abformung) as described in U.S. Pat. No. 6,068,684 to Overton, which is incorporated herein by reference. High-aspect-ratio channels can be easily formed in a wide variety of substrate materials using the LIGA-based techniques. Using a LIGA-based process to form a channel in a substrate, a thick layer of positive photoresist (e.g., PMMA) can be exposed to the x-ray beam through a patterning mask. The exposed areas of the photoresist can then be developed to provide a mold of the channel. If the mold is to be filled by electroforming, the photoresist mold can first be coated with a plating base. A structural material can then be electroformed on the plating base to fill the mold and form the walls of the channel. The rough, electroplated free surface of the filled mold can then be planarized by diamond lapping or the like. The remaining resist mold material can then be dissolved away to provide a hollow channel in the electroformed substrate. Typical LIGA-based column metals include copper and nickel. However, a wide variety of column materials can be deposited using LIGA processes, including gold, tantalum, chromium, aluminum, titanium, iron, metal alloys, and silicon.
Typically, the inside surfaces of the microfabricated channel is coated with a stationary phase material, such as a polymer, to enhance the separation of the chemical analytes of interest in the gas sample. However, the rectangular geometry is difficult to coat with a satisfactorily uniform stationary phase using conventional solvent evaporation or solution-coating methods. Sagging and pooling results in buildup of the stationary phase in the corners of the rectangular channel. This leads to a lower coating efficiency and tailing of the elution bands.
The evolution of portable, handheld microanalytical systems requires new stationary phase materials and coating methods that are compatible with new column materials and chromatography applications. In particular, these column materials require unique and specialized chemical functionalization to prepare and wet the column surface and to bond a uniform stationary phase to the column wall.